Welded joint and method of making the same



Feb. 15, 1944. R. E. HERDMAN 2,341,716

WELDED JOINT AND METHOD OF MAKING THE SAME Filed Jan. '7, 1943 Thor/hfed fungsfen Mame/77* mum/um fab P/af/num fab Tungs fen lead INVENTOR ROBERT ls. HERD/m HIS ATTORNEY Patented Feb. 15, 1944 WELDED JOINT AND METHOD OF MAKING THE SAME Robert E. Herdman,

or to Eitel-McCullough a corporation of Calif Salt Lake City, Utah, assign- Inc., San Bruno, Calif., ornia Application January 7, 1943, Serial No. 471,629

2 Claims.

My invention relates to the art of welding, and more particularly to the welding of delicate parts such as found in cathode structures of electron tubes.

It is among the objects of my invention to provide an improved method of welding parts together, especially adapted for securing a filament to its lead.

A further object includes the provision of a welded joint which has superior strength, and which protects the filament against failure due to breakage at the weld.

The invention possesses other objects and features of advantage, some of which, with the foregoing, will be set forth in the following description of my invention. It is to be understood that I do not limit myself to this disclosure of species of my invention, as I may adopt variant embodiments thereof within the scope of the claims.

Referring to the drawing:

The figure is a fragmentary side elevational view of a cathode embodying the improvements of my invention.

In terms of broad inclusion, my improved joint comprises metallic tabs interposed between the joined parts at the weld, one of the tabs being of platinum and the other of a metal such as tantalum which absorbs platinum. The method comprises interposing the tabs between the parts to be welded, and subjecting the joint to welding heat. In a cathode structure the platinum tab is preferably first spot Welded to the cathode lead, then the tantalum tab spot welded to the platinum, then the filament welded to the tantalum tab.

In greater detail, and referring to the drawing, the cathode structure illustrated comprises one or more filament coils 2 having depending ends 3. Two coaxial coils are illustrated, although it is understood that a single coil may be used. The filament is preferably of thoriated tungsten.

Ends 3 are arranged closely together and are welded to a cathode lead 4, preferably a tungsten rod. This lead is sealed to a stem or other portion of an envelope in the usual manner and provides a conductive support for an end of the filament. The other end of the filament coils are also connected to a lead, but this is not included in the view since my invention is illustrated by the fragment shown.

Since a thoriated tungsten filament is diiiicult to weld to a tungsten lead, it has been the practice to interpose a piece of platinum between the parts at the Weld. Platinum welds or fluxes readily with tungsten. However, platinum tends to amalgamate with the tungsten, materially weakening the latter. This offers no difiiculty at the heavy tungsten lead which has ample strength, but it introduces a serious problem at the filament side of the weld where a fine wire is involved. Considerable filament breakage has been experienced due to weakening of the filament wire at the weld.

This problem is overcome in my improved structure by interposing a second metallic tab 6 between the filament and platinum tab 1 at the weld. The added tab 6 is of a metal having the property of absorbing or soaking up excess platinum. This sponge-like action prevents the platinum from reaching the filament in any harmful quantities. I have found that tantalum possesses the desired properties, and it is preferably used for tab 6 overlying the platinum.

The method is quite simple, involving first spot welding platinum tab 7 to lead 4, then spot welding tantalum tab 6 to the platinum, and then spot welding the filament to the tantalum tab. The cathode structure is then preferably heated by passing current through the filament in an atmosphere of hydrogen. This heating further knits the parts together at the weld. In the final weld the platinum and tantalum form an alloylike material having good fiuxing properties, and in which the platinum is confined to the regions of the weld by the absorbtive properties of the tantalum. The improved weld has good mechanical strength, even when several filament wires are welded at the same point, as shown in the drawing. These multiple coil filaments are at best not easy to weld securely, and my improved weld is particularly suited for this job.

While I have described by improved welded joint in connection with a cathode structure, it is understood that the improved welding procedure may be used advantageously in other places where parts of tungsten or the like are to be welded, especially where one of the parts is of filamentary character.

I claim:

1. A cathode structure comprising a metallic lead and a fine tungsten filament welded thereto, and a flux interposed between said lead and filament at the weld, said flux comprising a body of platinum next to the lead and a body of tantalum next to the filament.

2. The method of making a filamentary cathode, which comprises welding a platinum tab to a tungsten cathode lead, welding a tab of tantalum to the platinum tab, and then Welding a thoriated tungsten filament to the tantalum tab.

ROBERT E. HERDMAN. 

